The Meridional Mode and Tropical Pacific Decadal Variability in the CMIP5 Models

Type: Poster presentation

Venue: AGU Fall Meeting 2012


Jason C. Furtado; Emanuele Di Lorenzo (2012) The Meridional Mode and Tropical Pacific Decadal Variability in the CMIP5 Models. AGU Fall Meeting 2012, San Francisco, CA.

The Pacific Meridional Mode (MM), an intrinsic coupled ocean-atmosphere mode of variability in the tropical and subtropical Pacific Ocean, is tied to the precursor or excitation pattern of the El Niño-Southern Oscillation (ENSO). Like its Atlantic counterpart, however, the Pacific MM is likely also linked to interannual and decadal-scale variability of tropical Pacific sea surface temperatures (SSTs) and hence affects decadal-scale modulation of ENSO. Therefore, the representation of the Pacific MM in coupled climate models may impact the accuracy of simulated Pacific decadal climate variability and its associated global climate teleconnections.

In this study, the fundamental dynamics of the Pacific MM are investigated in observations and the historical scenario of coupled climate models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). Observations illustrate strong low-frequency (> 7 years) power in SSTs in the central tropical Pacific Ocean and in the subtropical North Pacific south of Hawaii, nearly co-located with the SST representation of the Pacific MM. Models that exhibit a clear signature of the Pacific MM and its relationship to ENSO are also characterized by more pronounced decadal variability in the tropical Pacific, reaffirming the hypothesis that the MM is an important mechanism driving low-frequency modulations of the tropical Pacific. Atmospheric variability associated with the Pacific MM is then studied. In the observations, the variance of the Pacific MM is dynamically tied to the North Pacific Oscillation (NPO), a dipole in sea level pressure in the North Pacific. This NPO teleconnection pattern, however, is not well reproduced in most CMIP5 models and thus leads to a decoupling between tropical and extratropical Pacific SST decadal variability in the models. Implications of these findings on future model evaluations and on the uncertainty in decadal climate forecasts are also discussed.